The present invention relates to a maskant system for preventing unwanted hydrogen fluoride gas attack on superalloys used in turbine engine components and to a method for cleaning such components using the maskant system.
Fluoride cleaning systems are used to remove unwanted oxides from surfaces and service induced cracks of turbine engine components, such as turbine blade airfoils, formed from nickel base superalloys prior to repairing the components. Hydrogen fluoride gas used in the cleaning treatment both depletes and intergranularly attacks the component surfaces and the exposed cracks, removing essential elements that form gamma prime nickel particles, leaving for some specific applications an undesirable gamma layer on the surface and along the cracks. This depletion layer on the base superalloy is typically between 0.0004 and 0.0009 inches. Presently acceptable levels of intergranular attack can be as high as 0.012 inches in some alloys and some types of turbine airfoils.
Those components that can tolerate depletion and intergranular attack from the fluoride cleaning can be repaired and returned to service. There are other components, due to their intended operating conditions, e.g. stress and temperature, in order to be subjected to a repair that requires fluoride cleaning, require minimal depletion and intergranular attack. A suitable maskant is needed to protect these components as well as some areas of the components during fluoride cleaning treatments.
Accordingly, it is an object of the present invention to provide a maskant system which minimizes depletion and which substantially eliminates intergranular attack.
It is a further object of the present invention to provide a maskant system as above which increases the number of repair cycles beyond current levels.
It is yet a further object of the present invention to provide a maskant system as above which provides structural integrity improvement by not having hydrogen fluoride gas attack in critical areas.
The foregoing objects are attained by the maskant system and the method of the present invention.
In accordance with the present invention, a maskant system for use in a fluoride cleaning system is provided. The maskant system broadly comprises a plurality of layers of a parting compound applied to a component surface which requires protection and a plurality of layers of chromium rich maskant applied over the parting compound layers for substantially preventing intergranular attack and for reducing any depletion zone. The parting compound comprises a mixture containing colloidal silica, de-ionized water, fused alumina grains, and alumina powders such as 100 mesh alumina powder, 325 mesh alumina powder, and/or calcined and low soda alumina powder. The maskant is comprised of a chromium powder mixed with a binder, a wetting agent, a thickening agent, and water. The maskant system of the present invention has particular utility in the cleaning of turbine airfoils formed from nickel-based alloys or cobalt-based alloys.
In accordance with the present invention, a method for cleaning a turbine airfoil broadly comprises the steps of applying from 2 to 6 layers of a parting compound to a surface which requires protection, applying from 2 to 6 layers of a maskant over the layers of parting compound, and subjecting the surface to a hydrogen fluoride cleaning treatment.
Other details of the fluoride cleaning masking system and the cleaning method of the present invention, as well as other objects and advantages attendant thereto, are set forth in the following detailed description.